1. Technical Field of the Invention
The present invention relates to a reduced hardware control circuit device with current loop for broad band Hard Disk Drive applications. More particularly, the invention relates to a device of the above type and comprising a current loop for detecting the current value which flows on the coils of a voice coil electric motor incorporated in said Hard Disk Drive and including a sensing resistance, connected in series with the motor and at least a driving driver for a relative coil of the motor.
2. Description of Related Art
As it is well known in this specific technical field, in the Hard Disc Drives at least a magnetic disk is provided which serves as storing support and a data reading and writing head which is supported at the end of an actuator arm.
This arm controls the position of the head by means of a small electric motor of the voice coil motor (VCM) type, which typically includes a coil associated with the actuator arm, and also one or more permanent magnets which produce a magnetic flux wherein the coil is immersed.
The application of a controlled current on the coil causes an interaction between the permanent magnets and the coil itself so as to drive the motor according to well known Lorentz relations.
The motor rotation allows to activate the actuator arm which, by means of a kinematic coupling, drives the head or heads above the surface of the disk.
With reference to FIG. 1, a classical control circuit device with current loop for a motor M of the voice coil type and for Hard disk drive applications is shown.
A person skilled in this art will appreciate the presence, in the current loop, of two drivers which drive the motor M coils, respectively with gain +Ap and −Ap, a sense amplifier Gs which fixes the current in the sensing resistance Rs, an error amplifier and an input buffer stage receiving a control signal from a digital/analog converter (DAC).
The aim of the control device is fundamentally that of detecting the value of the current which flows in the motor to feedback drive the coils of the motor itself according to the shifting need of the reading head arm.
In the scheme of FIG. 1 the resistances Ri and Rf which fix the loop gain and which, generally, for a good matching are external to the current loop, complete the loop. Also an external network Rc, Cc is provided which compensates the loop deleting the pole introduced by the motor.
In a current loop of this type the transfer function is fixed by the ratio Rf/Ri and by the values of Rs and Gs.
Making reference to the acronyms of the components shown in FIG. 1 what follows is obtained:I=(−Rf/Ri)*(Vin/Rs Gs)*(1/(1+s Rf(Rm+Rs)Cc/(2Rs Gs Ap))
FIG. 2 schematically shows how the loop band in this configuration depends, at a rough estimate, on the external components Cc, Rs and Rf and on the amplifiers Gs and Ap gains, besides on the motor resistance.
In particular a loop gain is given by the following relation:Gloop=(−1/(s Cc Rf))*(2Ap Gs Rs)/(Rs+Rm)With a corresponding passing band given by:ωt=(1/(Cc Rf))*(2Ap Gs Rs)/(Rs+Rm)
FIG. 3 shows the scheme of the implementation which is currently commercialized.
In this case the sense amplifier has been implemented with a stage Gm/Gm and with a local reaction which fixes the gain thereof for improving its reaction in the common input way. This solution is described in U.S. Pat. No. 6,072,339, to the same Applicant, the disclosure of which is hereby incorporated by reference.
Although advantageous under several points of view, this solution has some limitations since the system band is limited by the poles of the output stage, but also by those of the sense amplifier and by the error amplifier band, since they are all locally reactioned systems. Therefore, although changing the values of the capacitive and resistive components Cc and Rf external to the current loop, even in a suitable way, the system band, before or after, happens to approach these poles and thus to worsen the step margin.
There is accordingly a need in the art for devising a control circuit device for broad band Hard Disk Drive applications having such structural and functional characteristics as to improve the performances of the current loop typically used for driving the voice coil motor with the employment of a reduced number of circuit blocks in the control portion and a corresponding reduction of circuit area.
There is also a need in the art to increase the total band of the current loop eliminating possible local loops which limit the broad band performance.